Not Applicable
Not Applicable
1. Field of the Invention
This invention relates generally to the manufacture of printed circuit boards, and more particularly to an improved entry and exit material for drilling holes in printed circuit board panels.
2. Description of the Background Art
Printed circuit board panels are typically manufactured as multi-layered panels having one or more copper conductive layers separated by at least one electrically insulating layer frequently made from epoxy-glass resin. During fabrication of the board, many holes must be drilled through the board to allow component leads and. support structures to be secured onto the board. Drilling operations have historically created a certain number of defects in the final panel requiring additional work or the panel to be discarded. For example, entry and exit burrs often occur as a result of a build up of drill heat when using rotary drills. Sanding and de-burring equipment was developed to eliminate the burrs. However, drill burrs are often rolled back into the holes during processing of the board and provide a place for gases and solvents to, accumulate and may also preclude proper plating.
Excessive drill heat during the drilling process may also cause what is referred to as hole smearing. A hot drill bit may soften and liquefy the epoxy insulating material from the laminate and smear it through the hole. One solution to the smearing problem has been to apply a strong acid to etch back the smear. However, some material is resistant to acid etch back techniques.
Hole contamination may also occur from the deposition of contaminants by the heated drill bit into the hole. Likewise, a heated drill bit may also cause nail heading. Nail heading occurs when one copper layer deforms and protrudes into an inner copper layer. Nail heading occurs when, the drill bit accumulates material from poor quality laminates creating a dull and ineffective drill bit. The drill press will punch rather than drill through the copper layers thereby pushing a portion of the metal layers into the hole. Pressure feet, used to keep the panel in position, have also been known to create marks in the soft copper conductive layer by engaging the surface as well as driving drilling remnants into the surface of the board.
All of the aforementioned problems may lead to unreliable or unwanted electrical contacts in the circuit board leading to board failure. Accordingly, entry and back up materials were developed in an attempt to alleviate these problems. Entry materials were developed that are placed on the surface of the circuit board prior to drilling and exit materials were placed below the board.
Early entry materials were typically made of paper to protect the board from the pressure foot of the drill. Later entry materials included a thin foil made of aluminum as a heat sink. Phenolic resins were added to the paper layer to provide rigidity and stability to the entry material. Entry materials have been reasonably effective in reducing entry burrs and reduced drill wandering. However,the phenolic resins sometimes contributed to hole smearing and occasional nail heading and such resins are resistant to acid etching. Foil layers were helpful in reducing drill temperatures but the overall service life of the drill bit was still diminished somewhat by the additional cutting required by the bit.
Therefore, a need exists for an entry or exit material that will protect the surface of the circuit board, reduce drill bit wear and drill temperature, as well as reduce the incidence of burring, smearing, heading and drill wandering and the like. A need additionally exists for an entry or exit material that can be easily and inexpensively fabricated. The present invention satisfies those needs, as well as others, and overcomes the deficiencies of previous material designs.
The present invention comprises an entry material and an exit material for use in the drilling process, was well as a method of use. In general terms, the entry material of the present invention comprises a laminate sheet with a top metal foil layer, a center core layer, and a bottom foil layer and a method of use. The exit material of the present invention comprises a laminate of a foil layer or a paper layer and a fiberboard layer.
By way of example, and not of limitation, in accordance with one aspect of the invention, an entry material is provided with a pair of foil sheets. Each foil sheet is laminated to either side of a paper core with an epoxy/particulate lubricant mixture under cold or hot pressure. In one embodiment, graphite powder having a particle diameter of between approximately five microns and fifty microns is added to the epoxy adhesive before lamination. The overall entry laminate thickness is preferably between approximately 0.007 inches and approximately 0.016 inches in thickness. The preferred thickness of the exit material of the present invention is between approximately 0.040 inches and approximately 0.125 inches.
Accordingly, a descending drill bit will encounter a top foil layer, a first layer of graphite and epoxy adhesive, a fibrous core layer, a second layer of graphite and epoxy adhesive and a bottom foil layer. Normally, however, the fibrous core will absorb the adhesive and lubricant mixture during the lamination process so that there are no discrete layers of adhesive.
In another embodiment of the entry material, the particulate lubricant that is mixed with epoxy is polyethylene glycol, having a molecular weight of between approximately 600 and approximately 4000.
In yet another embodiment of the entry material, the bottom foil layer is omitted. In this embodiment, a single layer of foil is applied to a fibrous sheet with lubricant filled epoxy.
Similarly, the exit or back-up material has a single foil layer that is adhered to a fiberboard layer with a homogeneous mixture of epoxy and a particulate lubricant of graphite, polyethylene glycol or both. In one embodiment of the exit material, the fiberboard layer includes graphite or polyethylene glycol as a resin additive when the fiberboard is fabricated. The preferred embodiment of the exit material of the present invention has a single foil layer adhered to the front and, back sides of the exit material. The exit material is turned over after the first drilling with this embodiment to be used for the drilling of a second set of printed circuit boards.
An object of the invention is to provide an entry or exit material for drilling printed circuit boards that improves drilling accuracy and reduces drill wear and drill temperature.
Another object of the invention is to provide a laminate entry or exit material that has a lubricant in the adhesive that is evenly disbursed throughout the adhesive.
A further object of the invention is to provide an entry and exit material that reduces burring and smearing, provides for efficient chip removal, and protects the circuit board from damage from the pressure foot of the drill.
Yet another object of the invention is to provide an entry and exit material that is inexpensive to manufacture and easy to use.
Further objects and advantages of the invention will be brought out in the following portions of the specification, wherein the detailed description is for the purpose of fully disclosing preferred embodiments of the invention without placing limitations thereon.